Oh man, it's hard to chose since most of it have it's pros and cons that are understandable
Skypiea was chore yeah but the lore and the final battle saved it and it's peak, and Enel is prolly my fave villain
Punk Hazard was weird but it's mostly because of the soul swap that made it weird, rest was great and Law introduction and the build up for the conflict of Doflamingo to Kaidou was awesome
Fish-Man Island was mostly a show-up of the crews after the timeskip, Hody was meh but what made me like it is the background and the flashback of Fish-Man Island itself, especially how it shaped Mosjgard too, and Shirahoshi is a great heroine to me and the plot twist about her being Poseidon is still peak
I think I chose with Thriller Bark as the worst, the addition of Brook was great (and the flashback about his past was awesome), Zoro vs Ryuma was good, Kuma introduction was scary as heck, and Perona was hilarious, but otherwise it's the weakest arc, no awesome background of the place (it just a massive abandoned ship), Moria is pathetic, and it's crew was forgettable sans Perona, it's more of horror-thriller parody with action